1. Field of the Invention
The present invention relates to a computer system capable of connecting an expansion unit.
2. Description of the Related Art
Various types of expansion units have recently been developed to expand the functions of personal computers such as laptop computers. When a computer main body is connected to such an expansion unit, and power supplies are subsequently turned on in accordance with a predetermined sequence, a computer system constituted by the computer main body and the expansion unit is started.
The sequence of turning on the power supplies of the expansion unit and the computer main body connected thereto is complicated for an operator. If the power supplies are turned on in an incorrect sequence, an operation error or a circuit breakdown may be caused.
In a conventional computer system, a connecting state of an external I/O (input/output) device is checked by an initial diagnosis test (IRT test). For example, a register capable of read/write operations is arranged at an I/O port of each I/O interface, and data representing a connecting state of an I/O device is stored in the register in advance. In this case, when a connecting state of an I/O device is to be checked, the data in this register is read out. The I/O device, e.g., a printer, connected to the I/O port is recognized on the basis of the readout data.
In the above-described arrangement, no identical I/O port addresses should be set on the same bus. However, the port addresses of I/O devices are constant. Therefore, for example, the computer main body cannot be connected to the expansion slot of an expansion unit including an I/O interface which has the same I/O port address as that of the I/O interface of the computer main body because of the coincidence of the port addresses. Accordingly, if identical I/O port addresses are present on the same bus, an error occurs in the above-mentioned IRT test. Even if no error occurs in the IRT test, collision of data on the data bus is caused during execution of an application program. If an expansion unit is used by changing its port address, an application program cannot be operated.
Under the circumstances, a strong demand has arisen for a computer system which can be efficiently operated when a computer main body is connected to an expansion unit.